The instant invention relates to fluorescence microphotography and more particularly to a method for controlling exposure in fluorescence microphotography and a device for carrying out the method of exposure control.
In order to measure the exposure time for micro-photographic recordings, at least part of the light emitted by the microscope image must be deflected onto a sensor. Conventionally two different methods have been employed to measure exposure time. In the first method, prior to exposure of the photographic film, a mirror is swung into the beam path and directs the light onto the sensor. Then, in a manner similar to a reflex camera, the mirror is swung out of the beam path for the exposure time. This method is characterized in that the maximum quantity of available light is used both by the sensor to measure intensity to determine exposure time and is directed to the film during exposure.
The second known method uses a beam splitter which directs a portion of the beam to the sensor and at the same time directs the remaining portion of the beam to the photographic film plane. The beam splitter has a specified splitter ratio which enables the intensity of the portion of the beam incident upon the film to be calculated from the intensity of the portion of the beam incident upon the sensor. This method has the advantage that the light intensity can be measured continuously during the exposure of the film.
Each of the above described methods are disadvantageous when applied to fluorescence microphotography. In fluorescence microscopy a bleaching out of the preparation (i.e., the object caused to emit visible light as a result of the incidence of ultraviolet radiation) occurs over time. This "fading" or fading effect requires that a correction be made to the exposure time. As a result, the first conventional method cannot be effectively used since the exposure time is measured only once with respect to the initial intensity. This single measurement cannot take into account the fading effect and thus, a correction for the fading cannot be made. Alternatively, correction of the exposure time can be made when fluorescence microphotography is to be performed using the second method, since exposure is continuously measured using the beam splitter. However, since the beam splitter directs only a part of the available quantity of light to the film, according to the splitter ratio, long exposure times are required. Further, the imaging performance is reduced by passing the beam through the beam splitter.
A single device by which both of the above alternative methods can used is disclosed in International Patent Application No. Ser. No. 88/00,714. In this device, a beam splitter system with splitter surfaces of differing transmission/reflection ratios and with a supplementary fully mirrored surface is disposed in the photographic beam path so as to be transversely displaceable. A rotatable mirror is provided in the measurement beam path ahead of the light receiver. While this device provides for the optional use of either of the above two methods, the device still has the above described drawbacks with respect to fading effect when conducting fluorescence microphotography.
Therefore, it is an object of the present invention to provide a method and apparatus for controlling the exposure of a photographic film during fluorescence microphotography which overcomes the above described drawbacks by providing a fading correction while maintaining maximum light intensity for exposure and, thus, short exposure times.
It is another object of this invention to provide a method and apparatus which is able to correct for the fading effect associated with fluorescence microphotography while maintaining high quality imaging performance.